Polymerization

Scientists observed nature's methods of joining elements into chains and duplicated that natural process to produce macromolecules or polymers. Polymerization is the linking together of smaller units (monomers) into long chains. The repeating units (mers) of some polymer chains are identical, as in polyethylene[Figure 6-3(a)], polystyrene, and poly(vinyl chloride). Copolymers contain two different types of monomers, such as poly(vinyl chloride) mixed with vinyl acetate to produce poly(vinyl acetate), and terpolymers such as ABS (acrylonitrile-butadiene-styrene) contain three types of monomers. Isomers are variations in the molecular structure of the same composition. Isomers are found not only in hydrocarbons (HCs) but also in polymeric molecules. One class of isomers is known as stereoisomers, which, in turn, is divided into three categories. The first category is called isotactic stereoisomerism. Using a single carbon (C) chain of atoms with some hydrogen (H) atoms being replaced by another atom or group of atoms, denoted by the symbol R, one can demonstrate that the R group atoms are situated on the same side of the carbon chain. In a syndiotactic stereoisometric arrangement, the R group of atoms are located on opposite sides of the carbon chain in an alternating fashion. The third category is called atactic stereoisomerism, in which the R groups are in a random position. It is important to note that copolymers and terpolymers consist of units from each contributing mer and are not an alloy of mers. If polymerization permitted only the production of homopolymers, the properties of polymers would be severely limited.

Most polymers are produced by unsaturated hydrocarbons, which means that they have one or more multiple covalent bonds, such as ethylene:

or adipic acid, which is used in nylon synthesis and has two hydroxyl monomers:

Saturated hydrocarbons have all single bonds. To achieve the polymerization process, monomers must be capable of reacting with at least two neighboring monomers or be bifunctional. Because of copolymerization, terpolymerization, or other multicomponent polymerizations, a large variety of polymers is available. One example is styrene, a brittle polymer that has limited toughness and poor chemical resistance. But through copolymerization of acrylonitrile with styrene, it is possible to obtain a chemically resistant, more rigid, and stronger plastic. Copolymerization of butadiene with styrene yields an elastomer; and terpolymerization of acrylonitrile, butadiene, and styrene produces ABS plastics that are tough and elastomeric, and have good chemical resistance.

Catalysts begin the polymerization process. In addition, these chemicals serve a variety of purposes in chemical processing, ranging from serving as molecular sieves to agents for extracting heating oil, gasoline, and ethane from crude oil, to serving as a molecular matchmaker that holds chemicals together for a reaction to occur such as forming long-chain polyethylene polymers from molecules of ethane. Zeolites are catalysts composed of inorganic grains of alumina (AI₂O₃ ) and silica (SiO₂) through which molecules of crude oil are strained to gain oil by-products. Activated carbon, platinum, and nickel are also used to catalyze synthetics. Aerogels are open-cell foams that have ultrafine cell or pore size less than 50 nm, high surface areas (500 to 1000 m²/g), and density approaching that of air. They are the lightest, most transparent human-made solids. Known as the world's best insulators, they do not conduct heat. With a density of 15 mg/cm³ and a melting point of 1552^oF, the properties of these materials approach those of glass. Aerogels of organic and inorganic systems of fibrous chains provide catalyst support. The development of scanning tunneling microscopes (STMS) has allowed nanoscale examination of catalysts. Thereby, new designer catalysts can catalyze products more efficiently without creating polluting byproducts.

The main polymerization processes are addition (chain reaction) polymerization and condensation (step reaction) polymerization. The addition process is the simpler of the two. By use of heat and pressure in an autoclave or reactor, double bonds of unsaturated monomers break loose and then link up into a chain. These addition reactions (in addition polymerization for unsaturated HCs) are atoms or groups of atoms that attach themselves to the carbon atoms at the sites of multiple bonds. No products other than the polymer are formed. Saturated hydrocarbons undergo substitutional reactions in which hydrogen atoms are replaced by other atoms or groups of atoms. The products of addition polymerization, also referred to as chain reaction polymerization, include polypropylene (PP), polyethylene (PE), poly(vinyl acetate) (PVA), poly(vinyl chloride) (PVC), acrylonitrile-butadiene-styrene (ABS), and polytetrafluoroethylene (PTFE). Chain polymers generally fit into the thermoplastic (soften when heated) group. Most plastics that cannot be resoftened when heated (thermosetting polymers) come from condensation polymerization, also known as step reaction polymerization. This group gets its name from the by-product (condensate) of the polymerization, which is often water, but it may be a gas. Phenolic (PF), polyester (PET), silicon (SI), and urethanes (PUR) are typical thermosets from the step reaction synthesis, while nylon (PA) and polycarbonates (PC) are thermoplastic resins synthesized through step polymerization.

The term resin covers both solid and semisolid organic polymers. Resins are often considered as the uncompounded ingredients or monomers that are mixed but not yet polymerized. For example, thermosetting resins or pellets are molded into thermosetting plastic or elastomeric parts. Sometimes the term resin is used synonymously with plastics (e.g., acetal resins of acetal plastics, or thermoplastic resin instead of thermoplastic plastics).